1. Field of the Invention (Technical Field)
The present invention relates to methods and apparatuses for coating materials, and the products and compositions produced thereby, including filaments coated with diamond (D) and diamond-like carbon (DLC).
2. Background Art
The present invention permits the making of advanced composites by the deposition of diamond (D) and/or diamond-like carbon (DLC) on the surface of filaments of materials such as graphite and Kevlar. The D/DLC coating gives enhanced properties to the filaments and to composites formed from the filaments. Enhanced properties include improved compressive strength and improved thermal conductivity.
The deposition of D/DLC from plasmas containing carbon and hydrogen (e.g., H.sub.2 +CH.sub.4) onto flat surfaces is well known. Enhanced properties are known to result when the plasma bombards the workpiece with high-energy (e.g., 200 ev) ions during DLC deposition. DLC depositions are usually performed at low pressures (less than 0.1 Torr) to enhance ion bombardment. In contrast, D deposition is usually performed at higher pressures (many Torr) without ion bombardment, either using microwaves or radio-frequency power. Rudder et al, "Direct Deposition of Polycrystalline Diamond Films on Si(100) Without Surface Pretreatment," Appl. Phys. Letters 59:791 (1991).
What has not heretofore been known is the manufacture of composites fabricated from filaments coated uniformly with D/DLC. To be economical, the deposition should take place rapidly in a continuous-flow reactor that processes many filaments simultaneously. Filaments are fragile and normally are not handled individually, but are produced in bundles called tows consisting of 1,000 to 10,000 filaments. To achieve deposition on all sides of the filaments in a tow, the filaments must be spread apart during the deposition. The deposition rate of DLC is fast compared to D, but the economical production of DLC-coated tows still requires optimization of a production line with regard to deposition rate.
U.S. Pat. No. 4,402,993, entitled "Process for Coating Optical Fibers," to Aisenberg, et al., discloses the deposition of a diamond-like carbon film onto a single optical fiber. The purpose of the thin carbon film in the patent is to enable further deposition of a hermetically-sealed coating to protect optical fibers from contact with the atmosphere, not to apply a thicker carbon layer to achieve enhanced strength and thermal conductivity of structural filaments to be used to form composite structures.
U.S. Pat. No. 4,530,750, entitled "Apparatus for Coating Optical Fibers," to Aisenberg, et al., is substantially identical to the '993 patent.
U.S. Pat. No. 4,918,031, entitled "Processes Depending on Plasma Generation Using a Helical Resonator", to Flamm, et al., describes the use of helical resonators to generate plasmas for processing of semiconductor devices. The authors cite previous use of helical resonators for isotropic semiconductor etching pressures above 0.1 Torr. Flamm, et al., is restricted to semiconductor processing or related applications such as making multi-layer dielectric mirrors.